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Swiss researchers increase capacity by minding the gap

Scientists from the École polytechnique fédérale de Lausanne (EPFL) in Switzerland have shown how to achieve a dramatic increase in the capacity of optical fibres. The solution, published in Nature Communication's December issue, reduces the amount of space required between the pulses of light that transport data.

The EPFL’s Camille Brès and Luc Thévenaz have come up with a method for fitting pulses together within optical fibres, which reduces the space between pulses, and makes it possible to use all the capacity within the fibre. This opens the door to a 10-fold increase in throughput in telecommunications systems.

There have been several different approaches to the problem of supplying more throughput to respond to growing consumer demand, but they often require changes to the fibres themselves. The EPFL team took a different approach by looking at the fundamental issue of how best to generate the pulses that carry the digital data, which would not entail a need to replace the entire optical fibre network.

The breakthrough is based on a method that can produce what are known as ‘Nyquist sinc pulses’. ‘These pulses have a shape that's more pointed, making it possible to fit them together, a little bit like the pieces of a jigsaw puzzle lock together,’ explained Brès.

The idea of putting pulses together like a puzzle to boost the optical fibres' throughput isn't new, but the puzzle had never been solved before. The EPFL team used a simple laser and modulator to generate a pulse that is more than 99 per cent perfect.

The shape of pulse is determined by its spectrum. In this case, to generate the ‘jigsaw puzzle’, the spectrum needs to be rectangular, meaning that all the frequencies in the pulse need to be of the same intensity. The team used a concept known as a ‘frequency comb’ and succeeded in generating pulses with almost perfectly rectangular spectrum.

Because the technology is already mature, as well as 100 per cent optic, relatively cheap, and could potentially fit on a simple chip, the new pulses are expected generate much interest within the telecommunications-industry market in 2014.

Semiconductor foundry CST Global is leading a UK government-funded project that could substantially reduce the cost of manufacturing high-speed laser diodes for next-generation fibre-to-the-premises (FTTP) networks.

The market for passive optical networks (PON) is expanding rapidly as consumers upgrade their broadband connections to higher speeds. Indeed, this summer, CST Global said it had shipped more than 25 million lasers into PON markets worldwide. To meet the insatiable demand for bandwidth, the next generation of PON lasers will need to be both higher speed and lower cost.